It is useful to look at the workings of a healthy urinary system to understand and appreciate what happens when it starts to falter.

The urinary system consists of the kidneys, bladder, ureters, urethra and the pelvic floor.

Kidneys

The kidneys produce urine at a rate of approximately 60ml per hour (1500ml in 24 hours).  The amount we pass will vary with what or how much we drink, the climate in which we live, medication and disease.

Transport to the bladder

Urine is transported from the kidneys to the bladder via the ureters.  The pressure in the ureter is slightly higher than in the bladder. 

The ureteric valves prevent the urine from reflux (backing-up) to the kidney. 

Once the urine enters the healthy bladder there is no chance of reflux.  The most common cause of reflux is congenital abnormality.

The bladder is a muscular bag, consisting of layers of smooth muscle.  Its capacity for holding fluid varies between individuals, an adult's bladder normally holding up to one and a half pints of urine when filled to capacity.

Most people feel the need to urinate (micturate) when the bladder contains just a half a pint, due to the messages sent to the brain as the bladder gently fills.  The purpose of the bladder is to store and expel urine.

The functional bladder varies considerably between individuals.  As we have control over when we empty our bladder, the signal determining functional capacity will be set by our individual interpretation of fullness or moulded habit.  Functional capacity can vary from 100ml to 400ml+.  

The better the functional capacity, the more hours we have between the need to void (pass urine).

The experience

Under normal conditions, urination is under voluntary control of the nervous system (sympathetic nervous system). 

During the filling phase of the bladder, the detrusor muscle stays relaxed under the influence of the sympathetic nervous system to accommodate the urine.  The bladder is compliant and will go on filling without a rise in pressure. 

It is important to understand that the filling of the bladder is not like the filling of a balloon where the pressure increases with volume; the bladder has the marvellous ability to fill without significantly increasing pressure.  Messages are sent to the brain letting us know about bladder fullness. 

How do we know when we need to empty our bladder? 

We sense an awareness in the pelvic region over the bladder (situated just behind the pubic bone).  We then make a decision to go the toilet - or not! 

If we are otherwise engaged at work or play we choose to ignore this warning.  We can do this because we are able to inhibit bladder contraction - effectively apply the brakes.  The bladder will not empty until we choose. 

When we stop inhibiting the bladder, contraction commences and at the same time the bladder neck opens to allow the urination.  However, as time passes the warning will get stronger until we need to take action and respond by going to the toilet.

Nonetheless, should you not be able to go, your bladder muscle will stay relaxed - there will be no reflux, meaning that the kidneys cannot be damaged by delaying urination. 

Our control is so perfect that should you be held up for some time in a traffic jam, for example, you will not be in danger of wetting your car seat.

Alternatively, at this point of holding on, you may set off for the toilet with a full bladder, and you are met with a queue.  This is where you may experience a rise in bladder pressure because you were expecting to void; the bladder muscle contracted (under the influence of the parasympathetic nervous system) and quickly you have to regain control. 

Note the sensation when you saw the queue - a really intense, urgent desire to void.   You may like to consider what you would do while you waited - cross your legs, jig about, dance.  We do these things because they help relax the bladder - by putting gentle pressure on the pudental nerve that causes it to settle.

Structural support - the pelvic floor

The pelvic-floor muscles form a hammock of interweaving fibres - attached at the back to the coccyx (the tailbones at the base of the spine) and at the front to the pubis.

Think of a hammock slung between two trees.  One could pull it up or let it down from either end.  Our pelvic hammock can be drawn up at will when the muscle fibres are contracted or let down when they are relaxed.

The genitals and outlets of urethra and bowel pass through the hammock - the fibres divide to bypass them.  These gaps in the hammock are the areas of greatest strain.

When our forefathers adopted the upright posture for locomotion, work and play, the muscles counteracted the pull of gravity, supporting the internal organs. 

To offset the additional strain, we have developed two strap-like slings of muscles to give greater security.  One loops around the urethra and the genitals, the other around the junction of the lower bowel with the anus.  They should contract spontaneously. 

The bladder neck mechanism is not under voluntary control - it is effective when the bladder pressure remains low but if the pressure is raised, the pelvic-floor muscles act voluntarily, providing secondary support. 

These muscles play an important part in controlling the flow of urine.  When the stress pressure is raised on laughing, coughing, sneezing, lifting, or hitting a ball they should contract spontaneously. 

However, some women may find that after pregnancy their muscles have become weak and are unable to prevent urine escaping.

The pelvic-floor muscles also control wind and are involved in sexual enjoyment.  To read more about Pelvic floor exercises, click here.

20/05/2009